Water and Soil Corrosion Chapter 3: Part II Water and Soil Corrosion

Water All corrosion in water depends on water coming in contact with the containing metal surfaces. If the metal surfaces are hydrocarbon wetted, little or no corrosion will occur. This is why the inside walls of many crude oil storage tanks are not coated and do not corrode. Organic liquids can dissolve polymers (plastics), but they are generally benign to metals, except when they contain corrosive dissolved components. H2S gas dissolved in hydrocarbons can produce hydrogen embrittlement, but this is normally worse if water is also available.

Dissolved gases Corrosion cannot occur if some other chemical is not available to be reduced. The reducible species often comes from dissolved gases. Hot water condensate return lines in power plants are made from carbon steel. They only corrode if air, containing CO2 and oxygen enters the system, e.g., through defective seals or valves. Because these lines need to handle differing amounts of liquid depending on the power load, they are only wetted along the bottom (6 o'clock position). This corrosion is sometimes called condensate channeling in the power-generation business.

Dissolved gases A similar pattern occurs in horizontal gathering lines in oil production. Most wells produce a combination of oil and water. The water may separate to the bottom of the lines and cause corrosion if CO2 or oxygen enters the system. This produces the same pattern of channeling along the bottom of the line.

Dissolved gases common gases that accelerate corrosion:

Effects of Dissolved Salts Pure water (pH=7) is essentially non-ionic and is a very good insulator. Pure water is non-corrosive. Salt water is more corrosive than fresh water. When salt is added to water, the electrical conductivity increases and so does the corrosion rate, at the same time, the oxygen solubility decreases with additional salt concentrations and this limits the corrosion rate, because reduction of oxygen is the rate- controlling chemical reaction.

Effects of Dissolved Salts In sea water, the highest corrosion rates are in the splash zone, where the metal is frequently covered with air-saturated water. The relatively low corrosion rates in the tidal region are due to oxygen concentration cells between the air-saturated water in the tidal zone and the lower concentrations in the fully submerged zone below it. The tidal zone, high in oxygen, is cathodic to the fully submerged zone just below, which is anodic. As the water deepens, the oxygen concentrations decrease and so does the corrosion rate.

Effects of Dissolved Salts Corrosion Rate of Iron in Air-Exposed Water with Varying Salt (Sodium Chloride) Concentrations corrosion rates of steel piling in seawater.

pH In the pH range from 4–10, the metal surface is covered with rust, a partially protective passive film, but the corrosion rate is still high enough that corrosion control efforts such as protective coatings, cathodic protection, or corrosion inhibitors are often necessary. At higher pHs, the metal becomes covered with mineral scales which cover the surface and provide increasing protection. At low pHs, the acidic environment dissolves any protective films and exposes bare metal to corrosion. Zinc, aluminum, and cadmium are sometimes called the amphoteric coating metals because they have the same amphoteric properties of corroding at low rates (below carbon steel) in neutral environments and at higher rates in both acids and bases. Pure water becomes more ionic at elevated temperatures.

Corrosion rate of zinc in water with different pHs Effect of pH on the Corrosion Rate of Iron in Water at Room Temperature Corrosion rate of zinc in water with different pHs

Effects of Mineral Deposits The chemical species that form with CO2 depend on the pH of the system as shown below. The solubility of CO2 varies with both temperature and pressure. As both temperature and pressure change in oil wells, calcium-rich mineral deposits may form on the inside of the upward-flowing production tubing, which frequently produce more salty water. Calcium carbonate and calcium sulfate scales form on the inside of oil-well tubing and these scales protect the underlying metal from corrosion, but they also restrict fluid flow. Drinking water suppliers deliberately send their water from the treatment plant slightly “hard,” which means they are oversaturated in calcium and magnesium, which causes the water to deposit thin mineral scales on metal surfaces.

Effects of Mineral Deposits Carbonic Acid, Bicarbonate Ion, and Carbonate Distribution as a Function of pH Calcium Carbonate (Calcite) Calcium Sulfate (Gypsum)

Effects of Liquid Velocity Fluid velocities can affect corrosion rates: Excess velocity can lead to increased corrosion (erosion corrosion), which is discussed in Chapter 5. Emulsions, mixtures of immiscible liquids, can be noncorrosive if a hydrocarbon continuous phase surrounds a dispersed water phase. If the water phase is continuous, then corrosion will occur. If liquids become stagnant, then phase separation can occur and water will wet the surrounding metal container. In low flow areas, solids or sediment can drop out, causing under-deposit corrosion. Microbiologically influenced corrosion (MIC) is most often found in stagnant or low-flowrate liquid systems.

Effects of Liquid Velocity Corrosion Due to Water Separation at the 6 o'clock Position on a Low-Velocity Crude Oil Gathering Line

Effects of Temperature High temperatures generally increase chemical reactions, including corrosion reactions. High temperatures lower the solubility of dissolved gases. Pressure alters boiling points. Pressure vessels and downhole environments often have liquid water up to 250°C (400+°F). The degree of ionization of water depends on temperature, and this alters the pH. 

Effects of Temperature pH and ionization of water depends on temperature Changes in Oxygen Solubility in Water Exposed to Air at Various Temperatures

Microbiologically-Influenced Corrosion Microbiologically-influenced corrosion (MIC) and the bacteria that can produce MIC are classified in many ways. Bacteria can be classified as: Planktonic bacteria that freely float or “swim” in a body of water Sessile bacteria that are attached to surfaces and become motionless Many organizations collect water samples and determine the presence or absence of planktonic bacteria without recognizing that sessile bacteria, sampled by the use of insertion coupons or probes, is more important. Some of the most important types of bacteria associated with MIC are: • Sulfate-reducing bacteria (SRB) •Iron-oxidizing bacteria (IOB) •Acid-producing bacteria (APB) •Sulfur-oxidizing bacteria (SOB) •Slime-forming bacteria Figure 3.27 Pitting Under Microbial Deposits MIC is usually controlled by a combination of mechanical cleaning to remove surface biofilms followed by injection of biocides on a continuous or batch basis depending on the system in question. MIC is often associated with pitting corrosion, but it can be found in conjunction with virtually any corrosion form.

Microbiologically-Influenced Corrosion Some of the most important types of bacteria associated with MIC are: Sulfate-reducing bacteria (SRB) Iron-oxidizing bacteria (IOB) Acid-producing bacteria (APB) Sulfur-oxidizing bacteria (SOB) Slime-forming bacteria MIC is usually controlled by a combination of mechanical cleaning to remove surface biofilms followed by injection of biocides on a continuous or batch basis depending on the system in question. MIC is often associated with pitting corrosion, but it can be found in conjunction with virtually any corrosion form.

Microbiologically-Influenced Corrosion Pitting Under Microbial Deposits

Soils Water and gas occupy much of the space between solid particles of soil, and these are important in determining the corrosivity of soils. The air-soil interface is the most corrosive location for buried materials. Cathodic protection does not work on the loosely consolidated surface soil and corrosion control requires special attention— additional coatings and frequent inspections. Underground corrosion varies with soil types. The physical characteristics of soils affecting corrosion are primarily related to grain size and distribution, and moisture retention and aeration.

Soils In a soil that contains an uneven distribution of particle sizes or large rocks, differential environment corrosion cells can be created. Soils with otherwise benign characteristics can become very corrosive. Soil moisture and access to air frequently determine how much corrosion occurs on buried structures. The loose soil and air pockets at approximately 4 o'clock and 8 o'clock are the locations where air-saturated water will cause corrosion. Anaerobic bacteria are associated with stagnant water conditions, which are likely to occur in association with buried structures.

Corroded Pipeline at Air-to-Soil Interface Soils Radial locations where corrosion is most likely to occur on buried pipe. Corroded Pipeline at Air-to-Soil Interface

High-Temperature Environments High-temperature gases can react directly with metals and cause corrosion, even in the absence of an electrolyte. This type of corrosion, sometimes also referred to as dry corrosion, is discussed in Chapter 5. High-temperature corrosion does not require aqueous environments. High-temperature corrosion typically occurs in gaseous environments. High-temperature environments can react directly with metals and cause corrosion.